1. Field of the Invention
The present invention relates to a current detector used in a control system for controlling a motor for driving a vehicle such as an electric vehicle or a hybrid vehicle, or for use in another system which requires current detection.
2. Description of the Related Art
FIG. 4 is a block diagram showing the structure of a conventional current detector used in a control system for controlling a motor for driving a vehicle. In this figure, reference numeral 1 indicates a current sensor using a Hall element. The Hall element has a function of outputting a voltage which is proportional to (i) flowing current and (ii) generated magnetic flux density.
FIG. 5 is a block diagram showing the structure of the current sensor 1. In the figure, reference numeral 7 indicates a Hall element. Reference numeral 8 indicates a core which generates a magnetic flux when current flows through wire L. Reference numeral 9 indicates a power circuit which receives 12 V voltage, and converts it to 5 V voltage and outputs the converted voltage. Reference numeral 10 indicates an amplification circuit which is driven using the 5 V voltage output from the power circuit 9. When voltage v1 is input from Hall element 7 to the amplification circuit 10, the circuit 10 amplifies voltage v1 and also adds an offset voltage to the amplified voltage, and outputs the amplified voltage including the offset as voltage v2. Reference numeral 11 indicates a constant current generating circuit for supplying a constant current to the Hall element 7. The 5 V voltage output from the power circuit 9 is supplied to the constant current generating circuit 11 as a reference voltage, and the above constant current is defined by potential-dividing the reference voltage by using a resistor.
In FIG. 4, reference numeral 3 indicates an A/D converter which receives voltage input via terminal C from current sensor 1, and calculates the ratio of the input voltage to a reference voltage input from the power circuit 4, and converts the ratio to a digital value, and outputs the digital value. Here, the power circuit 4 converts input 12 V voltage to 5 V voltage and supplies the 5 V voltage to the A/D converter 3. Reference numeral 5 indicates a CPU (central processing unit) for receiving the above digital value from the A/D converter, and calculates and outputs the value indicating a target current flowing through wire L. Reference numeral 6 indicates a power circuit for driving CPU 5.
Below, the operation of the current detector (used in a control system for controlling a motor for driving a vehicle) having the above-explained structure will be explained.
When a 12 V voltage is input via terminal B to current sensor 1, the power circuit 9 converts the 12 V voltage to 5 V voltage and outputs the converted voltage to the Hall element 7 and the constant current generating circuit 11.
On the other hand, when a target current to be measured flows through wire L, magnetic flux is generated in core 8. When the magnetic flux is applied to Hall element 7, the element 7 outputs voltage v1, proportional to the magnetic flux, to the amplification circuit 10. The amplification circuit 10 amplifies the input voltage v1 and adds an offset voltage to it so as to obtain voltage v2 (0 V xe2x89xa6v2 xe2x89xa65.0 V), and outputs voltage v2 via terminal C to A/D converter 3.
When the A/D converter 3 receives voltage v2 via terminal C, the converter 3 calculates the ratio of v2 to the reference voltage (i.e., 5V) input via terminal A from the power circuit 4, and converts the calculated result to a digital value and outputs the digital value to CPU 5.
In order to improve the accuracy of the current sensor 1, a constant current (from the constant current generating circuit) should be stable, that is, should not be affected by the external environment. Accordingly, the voltage supplied to the constant current generating circuit should be accurate, and thus an accurately operable power circuit must be used as power circuit 9.
In addition, the A/D converter 3 should also accurately digitize the measurement value of the current sensor 1, and thus the power circuit 4 should also be accurate.
Therefore, conventionally, in order to accurately measure the target current, both the power circuits 4 and 9 must be accurate, thus the cost will be high.
In addition, the values of reference voltage (5 V) of the power circuits may not accurately be the same, thereby producing an error.
Furthermore, in the control of the motor for driving a vehicle, such an error of the current sensor certainly causes an error in the torque control of the motor. More specifically, the difference between the real value and the actually detected value functions as an error of the control value, so that overestimates or underestimates of the control may cause a shock to the vehicle, or excessive discharging or charging of the battery functioning as a power source.
Therefore, for example, the system must be designed in consideration of an error in the detected current; thus, the design may be limited or the system cost may be high.
In consideration of the above circumstances, an objective of the present invention is to provide a current detector used in a control system for controlling a motor for driving a vehicle, wherein accurate current measurement can be performed without using especially accurate power circuits (as explained above).
Therefore, the present invention provides a current detector used in a control system for controlling a motor for driving a vehicle comprising:
a detector (for example, a current sensor 21 in the following embodiment) for outputting a voltage corresponding to a target current, wherein the detector has a current detecting element (for example, a Hall element 27 in the following embodiment) for detecting the target current;
an amplifier (for example, an amplification circuit 30 in the following embodiment) for amplifying and outputting the output from the detector;
an analog-digital converter (for example, an A/D converter 23 in the following embodiment) for converting the output from the amplifier to a digital data; and
a power circuit (for example, a power circuit 33 in the following embodiment) for supplying an output voltage to both the detector and the analog-digital converter.
In a typical example, the detector comprises:
a Hall element (for example, a Hall element 27 in the following embodiment) functioning as the current detecting element;
a core (for example, a core 28 in the following embodiment) for applying a magnetic flux corresponding to the target current to the Hall element; and
a constant current generating circuit (for example, a constant current generating circuit 31 in the following embodiment) for supplying a constant current to the Hall element.
Preferably, the amplifier adds an offset voltage to the amplified voltage, and outputs the amplified voltage including the offset.
Also typically, the vehicle is a hybrid vehicle which uses the motor for assisting the output of an engine.
According to the present invention, the output voltage from a single power circuit is supplied to the detector for detecting the target current and to the analog-digital converter; thus, an especially accurate power circuit is not necessary, and an accurate measurement result can be obtained by using a cheap power circuit whose accuracy is relatively low.